Ultralow Thermal Conductivity and High-Temperature Thermoelectric Performance in n-Type K_2.5Bi_8.5Se₁₄

We studied the narrow band-gap (0.55 eV) semiconductor K_2.5Bi_8.5Se₁₄ as a potential thermoelectric material for power generation. Samples of polycrystalline K_2.5Bi_8.5Se₁₄ prepared by spark plasma sintering exhibit exceptionally low lattice thermal conductivities (κ_lat) of 0.57-0.33 W m^-1 K^-1 in the temperature range of 300-873 K. The physical origin of such low κ_lat in K_2.5Bi_8.5Se₁₄ is related to the strong anharmonicity and low phonon velocity caused by its complex low symmetry, large unit cell crystal structure, and mixed occupancy of Bi and K atoms in the lattice. High-resolution scanning transmission electron microscopy studies and microanalysis indicate that the K_2.5Bi_8.5Se₁₄ sample is a single phase without intergrowth of the structurally related K₂Bi₈Se₁₃ phase. The undoped material exhibits an n-type character and a figure-of-merit (ZT) value of 0.67 at 873 K. Electronic band structure calculations indicate that K_2.5Bi_8.5Se₁₄ is an indirect band-gap semiconductor with multiple conduction bands close to the Fermi level. Phonon dispersion calculations suggest that K_2.5Bi_8.5Se₁₄ has low phonon velocities and large Grüneisen parameters that can account for the observed ultralow κ_lat. The degree of n-type doping can be controlled by introducing Se deficiencies in the structure, providing a simple route to increase the ZT to ∼1 at 873 K.